1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method. More particularly, the present invention relates to a method of forming a gate dielectric layer.
2. Description of the Related Art
With the trend of more miniaturized metal-oxide-semiconductor (MOS) devices, the quality of a gate dielectric layer is highly required. For example, the leakage current must be as low as possible. When the gate dielectric layer is made of silicon oxide, most manufacturers will perform a nitrogen doping process to adjust the quality of the gate dielectric layer and minimize leakage. To prevent possible impact on the threshold voltage, the flatband voltage between the substrate and the gate and the reliability of the device, a shallow nitrogen distribution is preferred in the nitrogen doping process.
However, to minimize the leakage current of the device, a flatter nitrogen distribution curve from the surface of the gate dielectric layer to its interface with the substrate is preferred. The reason is that the shallow nitrogen distribution formed by the conventional process still causes a higher leakage current when the device is miniaturized.
The aforementioned nitrogen doping process is usually performed using nitrogen plasma. FIG. 1 is a graph showing the nitrogen distribution in a silicon oxide layer caused by a conventional nitrogen plasma treatment. Wherein the nitrogen gas flow rate is 200 sccm, a high frequency power is about 500 W and a pressure is about 80 mTorr. As indicated by the nitrogen distribution curve 100 in FIG. 1, the nitrogen dopants are mostly distributed within a shallow band of the silicon oxide layer to form a distribution peak 110.
The gate dielectric layer having a shallow nitrogen distribution can only suppress the electron tunneling effect to a limited extent. Because the nitrogen concentration in the lower part of the gate dielectric layer is rather low, more energetic electrons can still tunnel through to cause more leakage current.